The use of capacitive techniques provides a convenient and straightforward method of obtaining information from a security document, in particular information concerning the nature of the document which may include some measure of its authenticity. Unfortunately the nature of capacitive coupling means that problems are encountered in terms of the accuracy with which such information can be read. This is caused by limitations of the sensors used for this purpose and the different conditions under which such documents are interrogated, these including different contact conditions caused, in particular, by the condition of the security document. The outcome of these effects is that a problem occurs in the reliability of the identification of the documents. There are a range of practical situations in which this can very restrictive upon the usability of the technique. Thus the extent of wider adoption of capacitive sensing techniques upon security documents has a strong dependence upon advances in capacitive sensing technology. It is in this context that the present invention has been developed.